A pressurized water nuclear reactor comprises a tank which encloses fuel elements generating heat by the nuclear reactions that they produce. The zone of the fuel elements is surmounted by a zone enclosing the various control clusters for regulating the reactivity of the assemblies, and the various guidance members of these clusters. The suitable positioning of the clusters, more or less engaged between the fuel elements, is determined by actuating mechanisms outside the tank with, between the mechanisms and the clusters, connecting parts engaged in sheaths for traversing the cover of the tank. Generally the guidance and traversing sheath of the cover are terminated by a conical flared end-part in the vicinity of the upper support plate of the cluster guides.
Before starting up a nuclear power station, it is the rule to proceed with tests under pressure, when hot and when cold, by causing water to circulate in the primary circuit which, in service, is used to transfer the heat from the core of the reactor to the secondary exchanges. These tests are also used to detect and remove debris, turnings or filings which, in spite of the precautions taken during the assembly of the tank and of the primary equipment, can exist within the primary circuit.
French patents published under Nos. 2,280,178 and 2,413,757 describe filters used to trap such impurities, positioned on the lower plate of the core which, in service, supports the fuel assemblies. However, such filters, sometimes supplemented by flow measuring devices, result in pressure drops which are all the greater as the meshes of the filtering gauzes are finer. There is hence a limit to the fineness of the mesh of the filters located on the lower plate of the core, so that there can subsist within the circuits very fine particles of impurities which, precisely because of their fineness, can reascend to the upper parts of the tank, and therethrough be engaged in the traversal sheaths of the cover and reach the actuating mechanisms of the control rod clusters. To these particles passed through the filters of the lower plate of the core, particles detached downstream of the filters can be added, and these may also reascend to the sheath of the mechanisms and to the mechanisms themselves. This is very prejudicial to the safety of the operation of the reactor since, if the particles become deposited within the control rod mechanisms, they may cause subsequent faulty operation as a result of wear, jamming, or the like.